Channel description feedback in a communication system

ABSTRACT

A first communication device receives a plurality of training signal fields in a preamble of a data unit transmitted by a second communication device, the plurality of training signal fields in orthogonal frequency division multiplexing (OFDM) symbols comprising data and pilot tones. The first communication device determines first channel estimate data corresponding to the data tones in the OFDM symbols, and uses pilot tones in the plurality of training signal fields to track at least one of i) a phase offset and ii) a frequency offset while receiving the plurality of training signal fields to improve the first channel estimate data. The first communication device generates feedback data that i) includes data corresponding to the first channel estimate data, and ii) excludes data corresponding to second channel estimate data for the pilot tones in the plurality of training signal fields, and transmits the feedback data to the second communication device.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/859,993, filed Sep. 21, 2015, which is a continuation of U.S.application Ser. No. 13/207,003, filed Aug. 10, 2011, which claims thebenefit of U.S. Provisional Patent Application No. 61/372,376, filed onAug. 10, 2010, and U.S. Provisional Patent Application No. 61/407,705,filed on Oct. 28, 2010.

This application is also a continuation of U.S. application Ser. No.13/207,003, filed Aug. 10, 2011, which claims the benefit of U.S.Provisional Patent Application No. 61/372,376, filed on Aug. 10, 2010;and U.S. Provisional Patent Application No. 61/407,705, filed on Oct.28, 2010.

The disclosures of all of the patent applications referenced above arehereby incorporated by reference herein in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication networks and,more particularly, to channel description feedback in a communicationsystem.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Development of wireless local area network (WLAN) standards such as theInstitute for Electrical and Electronics Engineers (IEEE) 802.11a,802.11b, 802.11g, and 802.11n Standards, has improved single-user peakdata throughput. For example, the IEEE 802.11b Standard specifies asingle-user peak throughput of 11 megabits per second (Mbps), the IEEE802.11a and 802.11g Standards specify a single-user peak throughput of54 Mbps, and the IEEE 802.11n Standard specifies a single-user peakthroughput of 600 Mbps. Work has begun on a new standard, IEEE 802.11ac,that promises to provide even greater throughput.

SUMMARY

In an embodiment, a method includes: receiving, at a first communicationdevice, a plurality of training signal fields in a preamble of a dataunit, the plurality of training signal fields having been transmitted bya second communication device in orthogonal frequency divisionmultiplexing (OFDM) symbols comprising data tones and pilot tones;determining, by the first communication device, first channel estimatedata corresponding to the data tones in the OFDM symbols; using, at thefirst communication device, pilot tones in the plurality of trainingsignal fields to track at least one of i) a phase offset and ii) afrequency offset while receiving the plurality of training signal fieldsto improve the first channel estimate data; generating, by the firstcommunication device, feedback data that i) includes data correspondingto the first channel estimate data, and ii) excludes data correspondingto second channel estimate data for the pilot tones in the plurality oftraining signal fields; and transmitting, with the first communicationdevice, the feedback data to the second communication device.

In another embodiment, a first communication device comprises a wirelessnetwork interface having one or more integrated circuits configured to:receive a plurality of training signal fields in a preamble of a dataunit, the plurality of training signal fields having been transmitted bya second communication device via orthogonal frequency divisionmultiplexing (OFDM) symbols comprising data tones and pilot tones;determine first channel estimate data corresponding to the data tones inthe OFDM symbols corresponding to the plurality of training signalfields and second channel estimate data corresponding to the pilot tonesin the OFDM symbols corresponding to the plurality of training signalfields; generate feedback data that i) includes data corresponding tothe first channel estimate data, and ii) excludes data corresponding tothe second channel estimate data; and transmit the feedback data to thesecond communication device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example wireless local area network(WLAN), according to an embodiment.

FIG. 2 is a diagram of an example data unit format, according to anembodiment

FIG. 3A is a diagram of an example orthogonal frequency divisionmultiplexing (OFDM) symbol, according to an embodiment.

FIG. 3B is a diagram illustrating example pilot tone locations in anOFDM symbol, according to an embodiment

FIG. 4 is a table showing example feedback tone mappings for a 20 MHzchannel, according to various embodiments.

FIG. 5 is another table showing example feedback tone mappings for a 20MHz channel, according to various other embodiments.

FIG. 6 is a table showing example feedback tone mappings for a 40 MHzchannel, according to various embodiments.

FIG. 7 is a table showing example feedback tone mappings for 80 MHz and160 MHz channels, according to some embodiments.

FIG. 8 is another table showing example feedback tone mappings for 80MHz and 160 MHz channels, according to some other embodiments.

FIG. 9 is a flow diagram of an example method for transmitting channelestimate data from a receiver to a transmitter, according to anembodiment.

DETAILED DESCRIPTION

In embodiments described below, a wireless network device such as anaccess point (AP) of a wireless local area network (WLAN) transmits datastreams to one or more client stations. In an embodiment, a data streamtransmitted by the AP to a client station includes several trainingfields that allow the client station to develop a channel estimate (oran estimate of the channel response) describing the effects that thecommunication channel had on the transmitted data stream, and therebyallow the client station to accurately recover the transmittedinformation. Additionally, in some embodiments, the client stationtransmits a channel estimate in some form (includinguncompressed/compressed steering vectors, null space vectors, etc.) backto the AP. In some embodiments, the AP, in a technique known as explicitbeamforming, utilizes the received channel estimates to produce anantenna gain pattern having one or more lobes or beams (as compared tothe gain obtained by an omni-directional antenna) in the generaldirection of the client station antennas, with generally reduced gain inother directions. In other embodiments, the AP utilizes the receivedchannel estimates for a different purpose such as space-time encoding,precoding for spatial multiplexing, etc.

In some embodiments, such as embodiments utilizing multiple input,multiple output (MIMO) channels and/or orthogonal frequency divisionmultiplexing (OFDM), the amount of channel estimate data fullycharacterizing the communication channel (“a full channel estimate”) islarge. In embodiments utilizing multiple transmit and receive antennas(i.e., MIMO channels), for example, a full channel estimate includesestimates of the sub-channels corresponding to each transmit and receiveantenna pair. Further, in embodiments utilizing orthogonal frequencydivision multiplexing (OFDM), a full channel estimate includes channelestimates at each of the subcarrier frequencies. Therefore, to reducethe amount of channel estimate data transmitted from a client station tothe AP, in some embodiments, the client station transmits only a subsetof the full channel estimate data. For example, in some embodimentsutilizing OFDM-based communication, a technique of subcarrier groupingis utilized in which the OFDM subcarriers are combined into groups, andchannel estimate data corresponding to only one subcarrier in each groupis transmitted back to the AP. In some such embodiments, the AP 14, uponreceiving the subset, utilizes interpolation, duplication, or anothersuitable technique to generate channel estimate data corresponding tothe remaining subcarriers.

FIG. 1 is a block diagram of an example wireless local area network(WLAN) 10, according to an embodiment. An AP 14 includes a hostprocessor 15 coupled to a network interface 16. The network interface 16includes a medium access control (MAC) processing unit 18 and a physicallayer (PHY) processing unit 20. The PHY processing unit 20 includes aplurality of transceivers 21, and the transceivers are coupled to aplurality of antennas 24. Although three transceivers 21 and threeantennas 24 are illustrated in FIG. 1, the AP 14 includes differentnumbers (e.g., 1, 2, 4, 5, etc.) of transceivers 21 and antennas 24 inother embodiments. In one embodiment, the MAC processing unit 18 and thePHY processing unit 20 are configured to operate according to a firstcommunication protocol. The first communication protocol is alsoreferred to herein as a very high throughput (VHT) protocol. In anotherembodiment, the MAC unit processing 18 and the PHY processing unit 20are also configured to operate according to at least a secondcommunication protocol (e.g., the IEEE 802.11n Standard, the IEEE802.11g Standard, the IEEE 802.11a Standard, etc.).

The WLAN 10 includes a plurality of client stations 25. Although fourclient stations 25 are illustrated in FIG. 1, the WLAN 10 includesdifferent numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations 25 inother scenarios and/or embodiments. At least one of the client stations25 (e.g., client station 25-1) is configured to operate at leastaccording to the first communication protocol.

The client station 25-1 includes a host processor 26 coupled to anetwork interface 27. The network interface 27 includes a MAC processingunit 28 and a PHY processing unit 29. The PHY processing unit 29includes a plurality of transceivers 30, and the transceivers 30 arecoupled to a plurality of antennas 34. Although three transceivers 30and three antennas 34 are illustrated in FIG. 1, the client station 25-1includes different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers 30and antennas 34 in other embodiments.

In an embodiment, one or all of the client stations 25-2, 25-3 and 25-4has a structure the same as or similar to the client station 25-1. Inthese embodiments, the client stations 25 structured the same as orsimilar to the client station 25-1 have the same or a different numberof transceivers and antennas. For example, the client station 25-2 hasonly two transceivers and two antennas, according to an embodiment.

In various embodiments, the PHY processing unit 20 of the AP 14 isconfigured to generate data units conforming to the first communicationprotocol. The transceiver(s) 21 is/are configured to transmit thegenerated data units via the antenna(s) 24. Similarly, thetransceiver(s) 24 is/are configured to receive the data units via theantenna(s) 24. The PHY processing unit 20 of the AP 14 is configured toprocess received data units conforming to the first communicationprotocol, according to an embodiment.

In various embodiments, the PHY processing unit 29 of the client device25-1 is configured to generate data units conforming to the firstcommunication protocol. The transceiver(s) 30 is/are configured totransmit the generated data units via the antenna(s) 34. Similarly, thetransceiver(s) 30 is/are configured to receive data units via theantenna(s) 34. The PHY processing unit 29 of the client device 25-1 isconfigured to process received data units conforming to the firstcommunication protocol, according to an embodiment.

FIG. 2 is a diagram of an example data unit 250 that the AP 14 isconfigured to transmit to the client station 25-1, according to anembodiment. In an embodiment, the client station 25-1 is also configuredto transmit data units of the format of FIG. 2 to the AP 14. The dataunit 250 includes a preamble having a legacy short training field(L-STF) field 252, a legacy long training field (L-LTF) field 254, alegacy signal field (L-SIG) field 256, a first very high throughputsignal field (VHT-SIGA) 258, a very high throughput short training field(VHT-STF) 262, N very high throughput long training fields (VHT-LTFs)264, where N is an integer, and a second very high throughput signalfields (VHT-SIGB) 268. The data unit 250 also includes a data portion272. The data portion 272 includes service bits and information bits(not shown).

In an embodiment, the VHT-LTF fields 264 of the data unit 250 includetraining data that allows a client station to develop an estimate of thecommunication channel between the AP and the client station. The numberof VHT-LTF fields included in the data unit 250 generally corresponds tothe number of spatial channels via which the data unit 250 is to betransmitted, in various embodiments and/or scenarios. In someembodiments, however, the number of VHT-LTF fields included in the dataunit 250 exceeds the number of spatial channels via which the data unit250 is to be transmitted, at least in some scenarios. Further, accordingto an embodiment, as each VHT-LTF training field is transmitted to aclient station, the AP 14 applies a different mapping of symbols to thespatial streams thereby allowing the client station to develop a fullmultiple input multiple output (MIMO) channel estimate of thecommunication channel. Further still, in an embodiment utilizingorthogonal frequency division multiplexing (OFDM), the client stationdevelops a channel estimate corresponding to each of the subcarriertones.

FIG. 3A is a frequency domain diagram of an OFDM symbol 350 included inthe data unit 250, according to an embodiment in which the data unit 250occupies an 80 MHz bandwidth channel. In the illustrative embodiment ofFIG. 3A, the OFDM symbol 350 includes 256 tones (e.g., corresponding toa size 256 inverse discrete Fourier transform (IDFT)). The 256 tones areindexed from −128 to +127 and include guard tones, direct current (DC)tones, data tones, and pilot tones. The six lowest frequency tones andthe five highest frequency tones are guard tones. The three tonesindexed from −1 to +1 are DC tones. In an embodiment, the remaining 242tones are used as data tones and pilot tones. For example, in oneembodiment, eight of the 242 tones are used for pilot tones, andaccordingly, in this embodiment, 234 tones are used as data tones. Thedata tones at the highest absolute value indices (i.e., ±122 in theexample symbol 350) are referred to herein as “edge tones”. Data tonesin a training signal field (e.g., VHT-LTF field 264 in FIG. 2) arereferred to herein as “training data tones.”

FIG. 3B is a diagram illustrating example pilot tone locations in theOFDM symbol 350, according to an embodiment. As shown in FIG. 3B, theeight pilot tones occupy subcarrier indices {±103, ±75, ±39, ±11}. Inother embodiments, the eight pilot tones occupy other suitablesubcarrier indices. Further, in some embodiments, the OFDM symbol 350includes a different number of pilot tones, for example two pilot tones,four pilot tones, six pilot tones, ten pilot tones, or any othersuitable number of pilot tones, and the pilot tones are located at anysuitable subcarrier indices within the OFDM symbol 350.

In some embodiments, the data unit 250 occupies a channel that isdifferent than an 80 MHz bandwidth channel described above. For example,the data unit 250 occupies a 20 MHz channel, a 40 MHz channel, 160 MHzchannel or any other suitable bandwidth channel in some embodimentsand/or scenarios. In such embodiments, the OFDM symbols included in thedata unit 250 include different numbers of tones, and, accordingly,different numbers of tones are reserved for pilot tones as compared to adata unit that occupies an 80 MHz channel. For example, in oneembodiment, a 20 MHz OFDM symbol includes four pilot tones located atsub-carrier indices {±7, ±21}. In an embodiment, a 40 MHz OFDM symbolincludes six pilot tones located at sub-carrier indices {±11, ±25, ±53}.In other embodiments utilizing 20 MHz or 40 MHz channels, differentnumbers of pilot tones and/or different pilot tone locations thandiscussed above are utilized. For a 160 MHz OFDM symbol, in oneembodiment, the number of pilot tones and the pilot tone locations arechosen based on pilot tone numbers and positions in constituent 80 MHzOFDM symbols. In other embodiments, the number of pilot tones and thepilot tone locations for a 160 MHz OFDM symbol are chosen regardless ofthe number and locations of the pilot tones specified for an 80 MHz OFDMsymbol.

According to some embodiment, different pilot tone values are utilizeddepending on the OFDM symbol index and the spatial stream index. Thatis, in these embodiments, the pilot tones are defined as multi-streamtones. Various example transmission channels and pilot tone mappings andvalues that are utilized in some such embodiments are described in U.S.patent application Ser. No. 12/846,681, entitled “Methods and Apparatusfor WLAN Transmission”, filed on Jul. 29, 2010, which is herebyincorporated by reference herein in its entirety.

Referring again to FIG. 2, in an embodiment, as discussed above, eachVHT-LTF field 264 includes training data corresponding to eachsubcarrier frequency of an OFDM symbol, allowing the client station 25-1to determine a full MIMO channel estimate for each tone. On the otherhand, to improve accuracy of the channel estimate by eliminating orreducing errors associated with a frequency drift between the AP and theclient station, in some embodiments, some of the tones in the VHT-LTFtraining fields are used as pilot tones that allow frequency and/orphase to be tracked during reception of the training fields. However, inorder to implement frequency and/or phase tracking based on pilot tones,a client station generally requires knowledge of the channel responsecharacterizing the communication channel between the AP and the clientstation. In the case of a MIMO channel, a full MIMO channel estimate,therefore, needs to be known for accurate frequency and/or phasetracking. However, a full MIMO channel estimate generally cannot bedetermined until all of the training fields included in a data unit(e.g., all of the VHT-LTF 264 in the data unit 250 of FIG. 2) arereceived at the client station.

Accordingly, pilot tones included in the training signal field OFDMsymbols in some embodiments are “single stream” pilot tones, that is,the values of these pilot tones are defined based only on the OFDMsymbol index, and are independent of the spatial stream index. In theseembodiments, therefore, a client station determines a multiple input,single output (MISO) channel estimate for the pilot tones, and performsphase and/or frequency tracking using the MISO channel estimate.Consequently, in such embodiments, a client station is unable to obtaina MIMO channel estimate for OFDM sub-carriers corresponding to pilottone locations in the training signal fields.

As discussed above, in some embodiments, a client station determineschannel estimate data corresponding to each OFDM subcarrier and feeds atleast a portion of the channel estimate data back to the AP. Ideally,the feedback data includes channel estimates corresponding to each tonein an OFDM symbol. However, in some embodiments, due to, for example,wide channel bandwidths and/or large numbers of spatial streams,transmitting channel data for every subcarrier is, in some cases,unpractical and/or degrades performance. For example, in a case of arelatively fast varying communication channel, the amount of timerequired to transmit channel estimate data for every subcarrier exceedsthe amount of time during which these channel estimates are valid,according to one embodiment. Accordingly, to reduce the amount offeedback, in some embodiments, the client station 25-1 transmits channelestimate data corresponding to only a subset of the subcarriers. Forexample, according to an embodiment, the client station 25-1 feeds backchannel estimate data corresponding to one tone in a group of a numberof adjacent tones. The tones for which channel estimate data is fed backto the AP 14 are referred to herein as “feedback tones”. Similarly, thetones for which channel data is not fed back to the AP 14 are alsoreferred to herein as “non-feedback tones”.

In embodiments described below in which a client station cannotdetermine a full MIMO channel estimate corresponding to the pilot tonesof a training field OFDM symbol (e.g., the VHT-LTFs 264 in FIG. 2),channel data corresponding to the pilot tones is then not transmittedback to the AP 14. That is, in these embodiments, the feedback tones donot include subcarrier indices corresponding to pilot tone locations. Onthe other hand, to simplify receiver and/or transmitter implementation,in some embodiments, the feedback tones include edge tones and/or tonesadjacent to the DC tones in an OFDM symbol.

FIG. 4 is a table showing example feedback tone mappings for a 20 MHzchannel, according to various embodiments and/or scenarios in which theclient station 25-1 utilizes different subcarrier groupings to transmitchannel estimate data back to the AP 14. In an embodiment utilizing atone grouping of one (N_(g)=1), for example, feedback tones include alltraining data tones, and exclude the tones corresponding to the trainingfield pilot tone locations. In an embodiment utilizing a tone groupingof two (N_(g)=2), on the other hand, the client station 25-1 feeds backchannel data corresponding to only 30 tones (e.g., corresponding tosubcarrier indices indicated in the appropriate row in the table).Similarly, in an embodiment utilizing a tone grouping of four (N_(g)=4),the client station 25-1 feeds back channel data corresponding to only 16tones (e.g., corresponding to the subcarrier indices indicated in theappropriate row in the table). Similar to the example embodimentutilizing a tone grouping of one (N_(g)=1), in the example embodimentsutilizing tone groupings of two and four, the feedback tones do notinclude subcarrier indices corresponding to pilot tone locations.Further, in the embodiments of FIG. 4, feedback tones for the four tonegrouping case (N_(g)=4) includes only a subset of the feedback tones forthe two tone grouping case (N_(g)=2) (i.e., the feedback tones for thefour tone grouping case (N_(g)=4) does not include any tones that arenot included in the feedback tones for the two tone grouping case(N_(g)=2)).

FIG. 5 is a table showing another example feedback tone mappings for a20 MHz channel, according to various other embodiments and/or scenarios.The table of FIG. 5 is similar to the table of FIG. 4 except that in thetable of FIG. 5 the feedback tones in the lower and the upper sidebandsare symmetrical with respect to each other in every tone grouping (i.e.,N_(g)=1, N_(g)=2, N_(g)=4). As in the embodiments described above withrespect to FIG. 4, in the embodiments of FIG. 5, channel estimate datacorresponding to the pilot tone locations is not fed back.

FIG. 6 is a table showing example feedback tone mappings for a 40 MHzchannel, according to various embodiments and/or scenarios in which theclient station 25-1 utilizes different subcarrier groupings to transmitchannel estimate data back to the AP 14. As illustrated in FIG. 6, in anembodiment utilizing a tone grouping of one (N_(g)=1), similar to thecase of a 20 MHz channel discussed above, feedback tones include alltraining data tones, and exclude the tones corresponding to the pilottone locations. In an embodiment utilizing a tone grouping of two(N_(g)=2) in accordance with FIG. 6, the client station feeds backchannel estimate data corresponding to 58 tones (e.g., at the indicesindicated in the appropriate row of the table). Similarly, in anembodiment utilizing a tone grouping of four (N_(g)=4), the clientstation feeds back channel estimate data corresponding to 30 tones(e.g., at the indices indicated in the appropriate row of the table).Similar to the case of a tone grouping of one (N_(g)=1), in theembodiments utilizing tone groupings of two and four, the feedback tonesdo not include subcarrier indices corresponding to pilot tone locations.Further, similar to the embodiments of FIG. 4, in the embodiments ofFIG. 6, feedback tones for the four tone grouping case (N_(g)=4)includes only a subset of the feedback tones for the two tone groupingcase (N_(g)=2) (i.e., the feedback tones for the four tone grouping case(N_(g)=4) does not include any tones that are not included in thefeedback tones for the two tone grouping case (N_(g)=2)).

FIG. 7 is a table showing example feedback tone mappings for 80 MHz and160 MHz channels, according to some embodiments and/or scenarios. In anembodiment utilizing a tone grouping of one (N_(g)=1) for an 80 MHzchannel, similar to the 20 MHz and the 40 MHz cases described above,feedback tones include all training data tones, and exclude the tonescorresponding to the pilot tone locations. In an embodiment utilizing atone grouping of two (N_(g)=2) for an 80 MHz channel, the client stationfeeds back channel estimate data corresponding to 118 tones (e.g., atthe indices indicated in the appropriate row in the table). Similarly,in an embodiment utilizing a tone grouping of four (N_(g)=4) for an 80MHz channel, the client station feeds back channel data corresponding to62 tones (e.g., at the indices indicated in the appropriate row in thetable). In embodiments utilizing the above tone groupings for a 160 MHzchannel, the feedback carrier indices indicated in the table correspondto the constituent 80 MHz channels. Further, feedback tones for the fourtone grouping case (N_(g)=4) includes only a subset of the feedbacktones for the two tone grouping case (N_(g)=2) in some embodiments(i.e., the feedback tones for the four tone grouping case (N_(g)=4) doesnot include any tones that are not included in the feedback tones forthe two tone grouping case (N_(g)=2)).

FIG. 8 is a table showing another example feedback tone mappings for 80MHz and 160 MHz channels, according to some other embodiments and/orscenarios. The table of FIG. 8 is similar to the table of FIG. 7, exceptthat in the embodiments utilizing tone mappings of FIG. 8 the edge tonesas well as the pilot tones are excludes from the feedback tone subsets.Accordingly, the table of FIG. 8 indicates 232 feedback tones for a tonegrouping of one for an 80 MHz channel, 116 feedback tones for a tonegrouping of two for an 80 MHz channel, and 58 feedback tones for a tonegrouping of four for an 80 MHz channel. Similar to FIG. 7, inembodiments utilizing the above tone groupings for a 160 MHz channel,the feedback carrier indices indicated in the table correspond to theconstituent 80 MHz channels.

FIG. 9 is a flow diagram of an example method 900 for transmittingchannel estimate data from a receiver to a transmitter, according to anembodiment. With reference to FIG. 1, the method 900 is implemented bythe network interface 16, in an embodiment. For example, in one suchembodiment, the PHY processing unit 20 is configured to implement themethod 900. According to another embodiment, the MAC processing 18 isalso configured to implement at least a part of the method 900. Withcontinued reference to FIG. 1, in yet another embodiment, the method 900is implemented by the network interface 27 (e.g., the PHY processingunit 29 and/or the MAC processing unit 28). In other embodiments, themethod 900 is implemented by other suitable network interfaces.

At block 904, a receiver receives a plurality of training signal fieldsfrom a transmitter. In an embodiment, the training signal fields areVHT-LTF fields 264 in FIG. 2. Each of the training fields received atblock 904 includes one or more OFDM symbols, such as the OFDM symbol 350of FIG. 3A, according to an embodiment. The tones of the OFDM symbolinclude at least training data tone and pilot tones. The pilot tones areat locations illustrated in FIG. 3B, according to one embodiment. Inother embodiments, different numbers of pilot tones and/or differentpilot tone locations are used.

At block 908, the receiver determines channel estimate datacorresponding to the training data tones and the pilot tones in the OFDMsymbols received at block 904. In one embodiment, the channel estimatedata corresponding to the training data tones corresponds to a MIMOchannel estimate, while the channel estimate data corresponding to thepilot tones corresponds to MISO channel estimate.

At block 912, the receiver transmits channel estimate data correspondingto only a subset of the OFDM symbol tones, or data generated using thechannel estimate data corresponding to only a subset of the OFDM symboltones, back to the transmitter. In an embodiment, the subset excludesall of the pilot tones. That is, in this embodiment, channel estimatedata transmitted back to the transmitter excludes MISO channel estimatedata. In some embodiments, channel estimate data corresponding to onetone in every group of two adjacent data/pilot tones in the OFDM symbol,or channel data corresponding to one tone in every group of fouradjacent data/pilot tones in the OFDM symbol, is transmitted back to thetransmitter at block 912. In some example embodiments, the channelestimate data, or the data generated using the channel estimate data,transmitted back to the transmitter at block 912 corresponds to theindex locations illustrated in FIGS. 4-8. In one embodiment, the datagenerated using the channel estimate data is in the form of a steeringvector for steering signals from the transmitter in the direction of thereceiver. In another embodiment, the data generated using the channelestimate data is in the form of vectors that span a null space of thechannel estimate data.

At least some of the various blocks, operations, and techniquesdescribed above may be implemented utilizing hardware, a processorexecuting firmware instructions, a processor executing softwareinstructions, or any combination thereof. When implemented utilizing aprocessor executing software or firmware instructions, the software orfirmware instructions may be stored in any computer readable memory suchas on a magnetic disk, an optical disk, or other storage medium, in aRAM or ROM or flash memory, processor, hard disk drive, optical diskdrive, tape drive, etc. Likewise, the software or firmware instructionsmay be delivered to a user or a system via any known or desired deliverymethod including, for example, on a computer readable disk or othertransportable computer storage mechanism or via communication media.Communication media typically embodies computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism. The term“modulated data signal” means a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia includes wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, radio frequency,infrared and other wireless media. Thus, the software or firmwareinstructions may be delivered to a user or a system via a communicationchannel such as a telephone line, a DSL line, a cable television line, afiber optics line, a wireless communication channel, the Internet, etc.(which are viewed as being the same as or interchangeable with providingsuch software via a transportable storage medium). The software orfirmware instructions may include machine readable instructions that,when executed by the processor, cause the processor to perform variousacts.

When implemented in hardware, the hardware may comprise one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), a programmable logic device (PLD), etc.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, changes, additions and/or deletions may bemade to the disclosed embodiments without departing from the scope ofthe invention.

What is claimed is:
 1. A method, comprising: receiving, at a firstcommunication device, a plurality of training signal fields in apreamble of a data unit, the plurality of training signal fields havingbeen transmitted by a second communication device in orthogonalfrequency division multiplexing (OFDM) symbols comprising data tones andpilot tones; determining, by the first communication device, firstchannel estimate data corresponding to the data tones in the OFDMsymbols; using, at the first communication device, pilot tones in theplurality of training signal fields to track at least one of i) a phaseoffset and ii) a frequency offset while receiving the plurality oftraining signal fields to improve the first channel estimate data;generating, by the first communication device, feedback data that i)includes data corresponding to the first channel estimate data, and ii)excludes data corresponding to second channel estimate data for thepilot tones in the plurality of training signal fields; andtransmitting, with the first communication device, the feedback data tothe second communication device.
 2. The method of claim 1, wherein: thedata tones in the OFDM symbols are received via multiple spatialstreams; and the pilot tones in the OFDM symbols are single-stream pilottones.
 3. The method of claim 2, wherein the first channel estimate dataincludes multiple input, multiple output (MIMO) channel estimate datafor the multiple spatial streams.
 4. The method of claim 3, wherein thesecond channel estimate data includes multiple input, single output(MISO) channel estimate data.
 5. The method of claim 1, whereingenerating the feedback data comprises generating the data correspondingto the first channel estimate data to exclude at least some of the firstchannel estimate data.
 6. The method of claim 1, further comprisingdeveloping a channel estimate corresponding to i) each of the data tonesin the OFDM symbols corresponding to the plurality of training signalfields, and ii) each of the pilot tones in the OFDM symbolscorresponding to the plurality of training signal fields, wherein thechannel estimate includes the first channel estimate data and the secondchannel estimate data.
 7. A first communication device, comprising: awireless network interface having one or more integrated circuitsconfigured to: receive a plurality of training signal fields in apreamble of a data unit, the plurality of training signal fields havingbeen transmitted by a second communication device via orthogonalfrequency division multiplexing (OFDM) symbols comprising data tones andpilot tones, determine first channel estimate data corresponding to thedata tones in the OFDM symbols corresponding to the plurality oftraining signal fields and second channel estimate data corresponding tothe pilot tones in the OFDM symbols corresponding to the plurality oftraining signal fields, generate feedback data that i) includes datacorresponding to the first channel estimate data, and ii) excludes datacorresponding to the second channel estimate data, and transmit thefeedback data to the second communication device.
 8. The firstcommunication device of claim 7, wherein the first channel estimate dataincludes multiple input, multiple output (MIMO) channel estimate dataand the second channel estimate data includes multiple input, singleoutput (MISO) channel estimate data.
 9. The first communication deviceof claim 8, wherein the pilot tones in the OFDM symbols corresponding tothe plurality of training signal fields comprise single-stream pilottones.
 10. The first communication device of claim 7, wherein the one ormore integrated circuits are configured to generate the datacorresponding to the first channel estimate data to exclude at leastsome of the first channel estimate data.
 11. The first communicationdevice of claim 7, wherein the one or more integrated circuits areconfigured to develop a channel estimate corresponding to i) each of thedata tones in the OFDM symbols corresponding to the plurality oftraining signal fields, and ii) each of the pilot tones in the OFDMsymbols corresponding to the plurality of training signal fields,wherein the channel estimate includes the first channel estimate dataand the second channel estimate data.